Definition: You are given a question on college medicine. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: Mutations are errors in DNA that:
(A)are always harmful. (B)only occur in the presence of carcinogens. (C)increase tumour growth. (D)occur spontaneously at a low rate.

D